The present invention relates in general to interferometers and in particular to a new and useful ring interferometer which utilizes a particular arrangement and light source having a coherence time which is substantially shorter than a propagation time in a connected light path of the arrangement.
Ring interferometers, also called optical gyros, and their application for determining absolute, space dependent rotations while utilizing the Sagnac effect, are known (see for example "Review of Modern Physics" 39, No. 2 (1967), pages 475-493, "Sagnac Effect" by E. G. Post).
To measure the rotation rate, a light path is employed which encircles an area once or several times. The light is guided along its path by mirrors and/or light guides. It is particularly advantageous to use optical fibers for establishing the guide path, since then a large number of circular passages of the light can be provided.
Designs of ring interferometers are known from the literature (see for example "Fresnel-Fizeau Effect In A Rotating Optical Fiber Ring Interferometer", by V. Vali, R. W. Shorthill, M. F. Berg; "Applied Optics" 16, No. 10 (1977) pages 2,605-2,607). In prior art designs, disturbances which invalidate the results of measurements, occur in the ring interferometer, which are primarily caused by refractions in a light path (due to the Rayleigh dispersion, for example) and by polarization errors in the fiber. The disturbing effect of refractions is discussed in "Applied Optics" 18, No. 6 (1979), pages 915-931, "Sensitivity Analysis of the Sagnac-Effect Optical-Fiber Ring Interferometer" by S. C. Lin and T. G. Giallorenzi. The injurious effect of polarization errors is described in "Optics Letters" 4 (1979), pages 152-154, "Fiber-Ring Interferometer: Polarization Analysis" by R. Ulrich and M. Johnson. This latter article also deals with the interposition of polarization filters to reduce disturbances caused by polarization errors. To completely eliminate disturbances caused by polarization errors, it is necessary to provide a device for controlling the polarization. Polarization control devices have been described in "Applied Physics Letters" 35, No. 11 (1979) pages 840-842, "Polarization Stabilization On Single-Mode Fiber" by R. Ulrich and in the German patent application No. P 39 34 794.2 ("Method of Measuring Absolute Rotations And Device For Carrying Out The Method).